FIG. 3 is a circuit diagram showing a structure of a control apparatus for a conventional vehicle AC generator which is disclosed, for example, in Japanese Utility Model Registration Application No. 34900, 1989.
Operation of the conventional apparatus will be described by reference to this circuit diagram.
When a key switch 8 is closed upon starting of an engine, a base current flows to a power transistor 308 from a battery 7 by way of the key switch 8, a reverse-current blocking diode 11, a resistor 12 for initial excitation and a base resistor 309 of the power transistor 308, as a result of which the power transistor 308 is turned on, i.e., turned to a conducting state.
When the power transistor 308 is turned on, a field current flows to a field coil 102 from the battery 7 by way of the key switch 8, the reverse-current blocking diode 11, the resistor 12 for initial excitation, the field coil 102 and the power transistor 308, whereby an electric generator 1 is set to the state capable of generating electricity.
When engine operation is started, the generator 1 is rotationally driven, whereby generation of electricity is started. In this state, a divided voltage obtained by dividing a generated voltage of the generator 1 with voltage dividing resistors 301 and 302 is compared with a reference voltage derived from voltage division of a constant voltage supplied from a constant-voltage power source A with voltage dividing resistors 303 and 304 by means of a comparator 305 of a voltage regulator 3. When the divided voltage resulting from the division of the voltage generated by the generator 1 is equal to or lower than the preset reference voltage value resulting from the voltage division of the voltage of the constant-voltage power source A, the comparator 305 assumes a conducting or on-state from a non-conducting or off-state, which results in that a transistor 306 is turned off from the conducting state or on-state.
When the power transistor 308 is turned on, then the field current tends to flow through the field coil 102. However, because the comparator 305 is in the conducting state, the voltage smoothed by a smoothing circuit 4 is discharged to a discharging resistor 405 from a capacitor 404.
In this conjunction, it is noted that the discharging time constant determined by the capacitor 404 and the discharging resistor 405 is large. Consequently, the output of a comparator 601 assumes on- and off-state periodically at a predetermined interval until it is detected that the generated voltage of the generator 1 reaches a predetermined value through comparison of a triangular waveform voltage outputted from a triangular waveform generator 5 with the discharge voltage of the smoothing circuit 4.
As a result of this, a transistor 602 is turned on and off in response to the output of the comparator 601, whereby the power transistor 308 is turned on and off with a predetermined duty ratio to thereby control the field current flowing through the field coil 102 such that generation of the output current of the generator 1 is retarded.
When the output current of the generator 1 has reached a current level equivalent to a load current of a vehicle electric load 9 with the generated voltage of the generator 1 reaching a predetermined value, the output of the comparator 601 assumes on/off levels with such a duty ratio to produce the on/off control of the field current demanded for the output current of the generator 1.
The transistor 602 is turned on/off in response to the output of the comparator 601 to thereby control the power transistor 308 and hence the field current flowing through the field coil 102, as a result of which the generated voltage of the generator 1 is regulated to the predetermined value.
As is apparent from the above, in the case of the conventional apparatus, the resistance value of the charging resistor 402 for the capacitor 404 constituting a part of the smoothing circuit 4 is selected to be small while that of the discharging resistor 405 is selected large. Thus, the value of the time constant for electric charging is set short or small (e.g. not greater than 0.5 sec.), whereas the time constant for electric discharge is set long or large (e.g. not smaller than 0.5 sec.), whereby such operation is ordinarily realized that the output of the comparator 601 becomes substantially equivalent to the output of the comparator 305 which constitutes a part of the voltage detecting circuit.
FIG. 4 shows changes in the generator output voltage waveform in the state where the vehicle-onboard electric load (e.g. headlight) is turned on in response to the turning-on or closing of a switch 10 and changes in the conducting rate of the power transistor 308 in consideration of the discharge characteristic.
In the conventional apparatus such as described above, the charging time constant of the smoothing circuit is set short while the discharging time constant is set long, wherein each of the time constants mentioned above is always set to a fixed value. Consequently, the ration of conduction of the power transistor 308 increases progressively or gradually in dependence on the magnitude of the discharging time constant which is effective when the electric load 9 is turned on and finally reduces a predetermined conducting rate after lapse of a predetermined time. Thus, the generator output voltage falls once temporarily to .DELTA.V1 upon turning-on electric energizing of the electric load 9 and thereafter gradually increases in conformance with the conducting rate of the power transistor 308.
The conventional apparatus is certainly effective for minimizing a decrease of the engine rotation number (rpm) by gradually increasing the output power of the generator when the engine, by which the generator is driven, is in the idling state. However, because the operation of gradually increasing the generator output is effective equally in a high-speed rotation state where the engine output allows a margin, the output voltage of the generator always falls every time the electric load is turned on, which in turn brings about variation in the output states of the loads already set to the on-state such as variation in luminance of lamps installed on instrumentation panels, room lamps, etc. which is of course uncomfortable for the driver, giving rise to a problem.